The article addresses challenges and actions from interference between trawl gear and offshore pipelines. Interaction from twin trawlers and their clump weights presents a particular challenge for offshore pipelines, especially in free-spans. The design loads are typically derived from DNV-RP-F111 and are of such magnitude that protection by rock installation is often required. This is especially the case for small and medium sized diameter pipelines, and has an obvious cost aspect tied to it.
The paper investigates the detailed behavior of clump weights interfering with pipelines by breaking down the kinematics of the clump weight and its interaction with an offshore pipeline to a set of differential equations. The system of equations is solved numerically and verified with tests. Next, the response of the clump weight and pipeline is assessed for various parametric variations. This involves assessing the sensitivity of the response for variations such as: interaction with free spans versus flat seabed, effect of varying the pipeline diameter, effect of contact friction, effect of trawl drag and warp line stiffness, effect of clump weight rigging, and the effect of lateral dragging resistance of the pipeline. Finally, the clump weight response solver is applied in case study to provide the trawl pullover design load for an FE pipeline model. The response and consequence for the pipeline is then finally compared with the equivalent scenario generated from the DNV-RP-F111 design pullover load.